In most microwave tubes the interaction between the wave and the beam is broken down into two steps:                a first step: obtaining a grouping of the electrons in bundles, that is to say producing a modulation of density of the current of the beam at the rate of the microwave signal; and        a second step: placing the duly obtained bundles of electrons in a phase in which they are slowed down by the field in order to give up their energy to the wave.        
In the case of TWTs, the grouping of the electrons in bundles is obtained by placing the beam in the field of a travelling wave whose phase velocity is equal to the velocity of the electrons. In a moving reference frame, the electrons see the field of a standing wave. The electrons are slowed over one alternation and accelerated over the next. A bundle of electrons is formed around the phase for which there is a transition from an accelerator field to a decelerator field.
A conventional waveguide, of rectangular or cylindrical section, is not suitable for the interaction because the phase velocity of the wave which is propagated in this guide is greater than the velocity of light while the velocity of the electrons is less than the velocity of light. Furthermore, an electrical field parallel to the displacement of the electrons is essential although the fundamental mode of the rectilinear guides of rectangular or cylindrical section is at right angles to the axis of the guide. To obtain a phase velocity less than that of light, a special guide is required that is called slow waveguide or delay line. More often than not the delay line is a periodic line obtained by translating a basic cell. Such is the case of the helix, of the coupled cavity line, of the interdigital line, etc.
In the field of TWTs operating at millimetric wavelengths, a delay line called folded guide is often used. This line is obtained by periodically positioning rectangular waveguide sections at right angles to the axis of the beam, and by alternately linking the straight guide sections by flat E bends at 180°. The cross-sectional view of the folded guide has the form of a snake. The beam slip hole is situated in the middle of the straight rectangular guide section. The electrical field in the guide is at right angles to the long side of the guide, and therefore parallel to the displacement of the electrons, which makes it possible to modulate the beam. The electron is therefore displaced in the slip hole, emerges in the straight guide section where it is subjected to the action of the electrical field (interaction space), passes back into the slip hole and emerges in the next interaction space. The electron therefore sees the successive interaction spaces with a period equal to the pitch of the line whereas the geometrical period of the line is equal to twice the pitch. The length of the folded waveguide (straight part and bends) is determined for the phase-shift of the wave in the guide to correspond to the phase variation linked to the displacement of the electrons from one interaction space to the next.
This folded guide line represents an analogy with the line with cavities coupled by alternate irises if the straight rectangular guide section is likened to a cavity where the wave-beam interaction occurs, and the flat E bends are likened to the coupling irises (see FIG. 11a). The particular feature of this line is that the same dimension is imposed for the width of the cavity and the width of the iris (the long side of the rectangular guide), which means that the bandwidth cannot be adjusted.
It is known practice to produce delay lines as illustrated in FIGS. 1 to 5, which schematically represent the central plate production which is then placed between a bottom plate and a top plate making it possible to close the waveguide.
FIG. 1 represents a central plate 1, in which a slip hole 2 for the electron beam is drilled in the lengthwise direction of the central plate 1. The central plate 1 has a rectangular parallelepipedal form whose faces are parallel to the axis of the slip hole 2 and symmetrical in relation to the axis of the slip hole 2.
As represented in FIG. 2, an emerging slit 3, in the form of a snake, is produced in the central plate 1, or in other words over all the thickness of the plate 1, over most of the length of the central plate 1, having its folds or meanders in the widthwise direction of the central plate 1.
The machined central plate 1 is equivalent to two interleaved combs 4, 5, as illustrated in FIG. 3, linked at the ends (different hatchings). It is also an alternative technology for producing this line (by using two combs and two rules for positioning the combs). The pitch of the slit 3 is the distance between successive portions of the slit 3 (or successive holes) along the longitudinal axis. The geometrical period of the slit 3 is equal to twice the pitch.
The removal of material which accompanies the machining of the slit 3 of the central plate 1 releases the stresses which can be reflected by deformations of the central plate 1. Thus, a longitudinal displacement or a transverse displacement of one comb relative to the other can in particular occur, as illustrated respectively in FIGS. 4 and 5.
The longitudinal displacement of one comb relative to the other, as illustrated in FIG. 4, modifies the width of the slit 3 which is no longer regular. In its displacement along the axis of the beam, in the slip hole 2, an electron sees a short interaction space followed by a long interaction space (portions of the slit 3). The period of the folded waveguide, or in other words the period of the slit 3 seen by the electron beam, is no longer the pitch of the slit 3 but approximately doubled. There is therefore a biperiodicity which can be reflected by a strong mismatch and risks of oscillations.
The transverse displacement of one comb relative to the other, as illustrated in FIG. 5, is reflected by an offset of the slip tunnel from one tooth of one comb to the next tooth of the other comb. There is then biperiodicity and risk of oscillation. Furthermore, the alignment defect reduces the useful section for transporting the beam, because it induces offset portions of the slip hole 2, and is reflected by a greater interception of the electron beam, which limits the average power of the travelling wave tube using such a waveguide.
Furthermore, a combination of the problems induced by a longitudinal slip and by a transverse slip of the two combs relative to one another is also possible.
FIGS. 6 and 7 schematically represent a waveguide, respectively in an exploded view and in a cross-sectional view along the longitudinal axis of the central plate 1.
In the example represented, the waveguide comprises a central plate 1 provided with a beam slip hole 2, rectilinear in the same direction as the longitudinal axis of the central plate 1, and comprises a slit 3, machined through the central plate 1. A bottom plate 6 and a top plate 7 close the waveguide, the slit 3 having its folds in the widthwise direction of the central plate 1. In this nonlimiting example, the folds or meanders of the folded waveguide or slit 3 are in the form of notches, or rectangular.